ansa-metallocene derivatives : XVII. Racemic and meso diastereomers of group IV metallocene derivatives with symmetrically substituted, dimethylsilanediyl-bridged ligand frameworks. Crystal structure of R,S-Me₂Si(3-t-Bu-5-MeC₅H₂)₂ZrCl₂

Abstract

The formation of racemic and meso diastereomers of the disubstituted ansa-zirconocenes (CH₃)₂Si(1-C₅H₃-3-R)₂ZrCl₂ (R = t-butyl, trimethylsilyl, α,α-dimethylbenzyl and 1-phenylcyclohexyl) from dimethylsilanediyl-bridged, symmetrically substituted bis-cyclopentadienyl dianion derivatives and ZrCl₄·2THF has been studied under a variety of conditions. Rac/meso ratios close to 1 were obtained in all cases. Similar product ratios were observed for analogous titanium and hafnium complexes with R = trimethylsilyl. A significant increase in the rac/meso product ratio was observed when an additional substituent was present next to the dimethylsilyl bridge as in (CH₃)₂Si(1-C₅H₂-3-R-5-CH₃)₂ZrCl₂ (R = CH(CH₃)₂, C(CH₃)₃). It is suggested that intermediates with one η¹-bound ring ligand control the stereochemical course of the ansa-metallocene formation. The crystal structure of R,S-(CH₃)₂Si(1-C₅H₂-3-C(CH₃)3-5-CH₃)₂ZrCl₂ is reported.